The present invention relates to a transmission control system for a facsimile transceiver which allows even facsimile transceivers furnished with different buffer memory sizes and/or decoding rates different from encoding rates to hold communications within a short transmission time.
In facsimile communications, the higher the data transmission rate, the shorter the transmission time is. However, since the response rate is limited by a minimum scanning time designed for a receiver, i.e., a recording speed of a printer or like recording device, the transmission rate has a certain upper limit even if the modified Huffman (MH) system or similar encoding system is employed to transmit band-compressed data. It is therefore necessary to observe the minimum transmission time assigned to a transceiver at a receiving station and such has heretofore been implemented by the insertion of fill bits, for example.
Usually, a transmitter of a facsimile transceiver is made up of a reader such as a scanner, an encoder, a buffer memory, a modem operable as a modulator, and a system control unit, while a receiver is made up of a modem operable as a demodulator, a buffer memory, a decoder, a recorder such as a printer, and a system control. Assume that the buffer memory of the transmitter, or transmit buffer memory, has a memory size Mt, the encoder has an encoding rate Ve, the buffer memory of the receiver, or receiver buffer memory, has a memory size Mr, the decoder has a decoding rate Vd, and the modems commonly have a modem rate Vm.
Assume that two facsimile transceivers each having the above construction exchange picture data. In the transmitting transceiver, the reader develops binary black-white data representative of a picture to be transmitted (hereinafter sometimes referred to as raw data). The encoder, which may comprise a microprocessor, processes the binary data output from the reader with the MH principle. The MH data are delivered from the encoder to the buffer memory. The encoded data once stored in the buffer memory are continuously transferred to the modulator synchronized with modem clock, which is output from the modulator. The procedure described so far is controlled by the system control unit.
The other or receiving transceiver demodulates the incoming encoded data by means of the demodulator. The demodulated data are continuously applied to the buffer memory in synchronism with modem clock, which is output from the demodulator. The data stored in the buffer memory are decoded by the decoder into the original binary data which are then reproduced by the recorder.
The picture signal processing times of such facsimile transceivers are related with each other as will be discussed mathematically hereinafter.
The transmitting transceiver station controls the generation interval of raw data (picture information) per line to within a predetermined range so that the generation rate of raw data at the reader may not exceed a plotter rate of the recorder at the receiving transceiver. This is, the control occurs such that the I/O rate Tp (ms/l) of the receiver of the receiving transceiver and the generation rate of raw data per line at the transmitting transceiver are in a relation EQU Ts.gtoreq.Tp (1)
The control represented by the relation (1) is always performed between facsimile transceivers of the same type and is especially required for transceivers of different types. Also, where raw data are input from an external memory or the like, the rate of receiving the data supplied from the outside has to be controlled as represented by the relation (1).
As previously described, a facsimile transceiver of the described type has a minimum transmission time Tmin per line (s/l) which is predetermined to match with a kind of the transceiver of a receiving facsimile transceiver; a control is performed such that the transmission time remains longer than the minimum transmission time by, for example, inserting fill bits. To improve the transmission rate, it is necessary to increase the processing rate of the reader and that of the recorder as well as the modem rate. This naturally results in a higher production cost. For given processing rates of a reader and recorder and a given modem rate, the transmission time cannot be shortened and, therefore, the transmission rate cannot be increased unless the number of fill bits to be inserted is made as small as possible.
A higher transmission rate may be implemented by a prior art transmission control system in which a transceiver at a transmitting station is furnished with a reader capable of reading a bunch of "N" lines at a time, and a multi-line memory for temporarily storing data read by the reader, i.e., raw data before compression, while a transceiver at a receiving station is furnished with a recorder capable of recording "N" lines at a time, and a multi-line memory for temporarily storing reproduced data. In this system, a minimum transmission time Tmin.times.N is predetermined for each "N" lines and, only when the transmission time has become shorter than the minimum transmission time, fill bits are inserted. In this manner, the system inserts fill bits on the basis of each "N" lines to maintain the minimum transmission time Tmin instead of inserting fill bits on a line-by-line basis to maintain the minimum transmission time Tmin, thereby averaging the density of read data. The result is a decrease in the number of inserted fill bits and, therefore, an increase in the transmission rate. Concerning such as a transmission control system, the larger the number of lines N, the greater the transmission time shortening effect is. However, an increase in the number of lines N is unattainable without an increase in the capacity of the line memory for storing raw data and, hence, without an increase in memory cost.
Another known implementation for a higher transmission rate is a transmission control system which constantly monitors a relationship between a quantity of encoded data temporarily stored in a buffer memory at a transmitting transceiver, i.e. memory size Mt, and a quantity of decoded data stored in a buffer memory at a receiving transceiver, i.e. memory size Mr. When the receive buffer memory is about to overflow, the system informs the transmitting transceiver of that storage condition to cause it to stop encoding operation or insert fill bits, thereby increasing the transmission rate. In detail, the system control is such that when the quantity of encoded data in the buffer memory at the receiving transceiver has exceeded a first value M.sub.1 (M.sub.1 &lt;Mr), the encoding operation is interrupted and, when the quantity of data has decreased beyond a second value M.sub.2 (M.sub.2 &lt;M.sub.1), the encoding operation is resumed and, when the data quantity has decreased beyond a third value M.sub.3 (0&lt;M.sub.3 &lt;M.sub.2), fill bits are inserted.
In the second prior art transmission control system, a response from the receiving transceiver has to be sent before the receive buffer memory overflows, in order to prevent the overflow. Again, this increases the costs becaust exchange of such responses between remote transceivers is unachievable without employing a signal generator or a receiver or installing an additional channel for the transmission of the responses.
As discussed above, the first-mentioned prior art transmission control system, which increases the transmission rate when the processing rates of a reader and recorder or the modem rate is constant, cannot achieve the purpose unless an expensive line memory having a large capacity is used. The second-mentioned prior art system requires the transmitting transceiver to send out information indicative of overflow of its buffer memory, increasing the system cost for the generation and exchange of response signals.